The field of art of this invention is aqueous modified epoxy resin dispersions which are particularly useful for can coating compositions. In particular, this invention pertains to aqueous dispersions made from a carboxyl-functional acrylic copolymer, phenolic resins, an epoxy resin partially esterified with a monofunctional acid, and, optionally, an epoxidized hydrocarbon or unsaturated vegetable oil.
In the manufacture of metal containers, a thin, protective synthetic coating is applied to the interior of the container. The synthetic resins employed for coating the interior of the metal container are heat-curable polymeric compositions which are applied in the form of a solution or dispersion in a volatile organic solvent or water. When organic solvents are used, there is the problem of contending with the hazards of solvent vaporization and recovery during the drying and baking cycles of a coating operation.
The can manufacturing industry utilizes cans which are fabricated from aluminum or steel which may or may not be coated with tin. The interior of the cans are coated with a thin thermoset film to prevent contact of the interior metal surface of a can with its contents. Such coatings ideally should have good adhesion to the interior metal surface, low extractables to prevent contamination of the container contents and a rapid cure rate for economy of container manufacture. Typical synthetic resin coating compositions include vinyls, polybutadiene, epoxies, alkyd/aminoplast and oleoresinous materials.
There is a continuing effort to utilize coating compositions dispersed or dissolved in aqueous media in order to reduce or eliminate the emission of organic solvents to the atmosphere.
U.S. Pat. No. 3,118,848 describes coating compositions which are prepared by mixing together a water-soluble salt of a vinyl polymer and a water-soluble epoxy or polyhydroxy compound. One or more water-soluble phenolaldehyde or amino resins, notably water-soluble urea-aldehyde or melamine-aldehyde resins may optionally be included as curing agents where low baking temperatures are contemplated.
In U.S. Pat. No. 3,492,252, an epoxy resin is dispersed in an emulsion of a carboxyl-containing vinyl or acrylic copolymer. Additional film-forming materials, e.g., phenol/formaldehyde resins, can be added to improve coating properties.
U.S. Pat. No. 3,969,300 describes coating compositions made by copolymerizing epoxy resins partially esterified with acrylic acid with other monomers including acrylic acid. These copolymers are solubilized or dispersed in water with amines and are then blended with amino resins.
U.S. Pat. No. 4,247,439 describes water-borne reaction products of carboxyl-functional polymers, polyepoxides and tertiary amines which are useful as film-forming components of coating compositions. Such coating compositions can be sprayed, flowed, dipped, rollercoated or electrocoated. The coating compositions are useful as such or can be crosslinked with crosslinking agents such as nitrogen resins and phenolic resins.
U.S. Pat. No. 4,283,428 discloses metal containers having an internal surface coated with a cured layer of a water based coating composition made from an aqueous medium having dissolved or dispersed therein a neutralized reaction product of a polyepoxide and an aromatic amino acid containing at least one amine group and at least one carboxyl group. The reaction product has unreacted carboxyl groups which are neutralized with a base. Such compositions are crosslinked with aminoplast resins and phenolic resins.
U.S. Pat. No. 4,289,811 describes aqueous dispersions of mixed resins which are prepared by polymerizing in the presence of an aminoplast resin a mixture of polymerizable carboxylic acid monomers and other monomers followed by the addition of an epoxy resin, ammonia or amine to salt the acid groups, and then dispersing the resin mixture in water.
U.S. Pat. No. 4,423,165 describes water-borne reaction products of carboxyl-functional polymers, polyepoxides and tertiary amines which are blended with carboxy-functional polymers having an acid number dissimilar to that of the first carboxy-functional polymer. The resulting products are useful as film-forming components of coating compositions, especially useful for coating the insides of two piece tinplated steel cans of beverage and food. The coating compositions are useful as such or can be crosslinked with nitrogen resins or phenolic resins.
In U.S. Pat. No. 4,454,265, aqueous thermoset coating compositions are described which are made from amine adducts of epoxy resins cured with heat hardening phenol-formaldehyde resins.
U.S. Pat. No. 4,212,781 describes graft polymers made by reacting epoxy resins with copolymerizable monomers in the presence of at least 3% benzoyl peroxide. These graft copolymers, when dispersed in water, form useful coating compositions for cans.
European Patent Applications Nos. 6334 and 6336 describe resinous compositions made from epoxy esters copolymerized with acrylic acid and other monomers. The epoxy esters are the reaction products of polyepoxide resins and acrylic acid. Before reaction with the acrylic acid, the polyepoxide resins can be partially defunctionalized with phenols or saturated monocarboxylic acids. The resulting resinous compositions are salted with a base and are dissolved or dispersed in water to form coating compositions. Aminoplast or phenolplast resins can be added to enhance the cure of the coatings on baking.